Fix/fix development error with react strict mode

[BorjaRafolsMartinez]

Hello.

This fixes a bug that happens when using react-hook-form with defaultValues option.

https://beta.reactjs.org/learn/synchronizing-with-effects#how-to-handle-the-effect-firing-twice-in-development

It basically makes sure it only runs once using the good old _mounted ref.

Should fix: https://github.com/tiaanduplessis/react-hook-form-persist/issues/18

I also added a test.

[BorjaRafolsMartinez]

@tiaanduplessis

[pedro757]

this works for me, thank you, I believe it should be merged

[JGJP]

Can we merge this please? I would like to have this soon otherwise will have to patch it manually

[JGJP]

tbh I think this PR makes a bunch of changes that are unrelated to the original issue